An electronic imaging system depends on a lens system to form an image on an electronic image sensor to create an electronic representation of a visual image. Examples of such electronic image sensors include charge coupled device (CCD) image sensors and active pixel sensor (APS) devices (APS devices are often referred to as CMOS sensors because of the ability to fabricate them in a Complementary Metal Oxide Semiconductor process). A sensor consists of a two-dimensional array of individual picture element sensors, or pixels. Each pixel is typically provided with either a red, green, or blue filter, as described by Bayer in commonly assigned U.S. Pat. No. 3,971,065 issued Jul. 20, 1976, so that a full color image can be produced. Regardless of electronic technology employed, e.g., CCD or CMOS, the pixel acts as a bucket in which photoelectrons are accumulated in direct proportion to amount of light that strikes the pixel during the capture of an image by the electronic imaging system.
Not all of the light that enters the front optical element of an electronic imaging system strikes a pixel. Much of the light is lost when passing through the optical path of the electronic imaging system. Typically, about 5% of the light is lost due to lens reflections and haze and about 60% is lost because of the color filter array. Moreover, some of the light strikes areas of the pixel that are not light sensitive. To gather the amount of light that is necessary to make a correct exposure, the electronic imaging sensor must gather light for an interval of time called the exposure time. Based on brightness measurements of the scene to be imaged, the electronic imaging system, typically an automatic exposure control, is employed to determine a suitable exposure time that will yield an image with effective brightness. The dimmer the scene, the larger the amount of time the electronic imaging system must gather light to make a correct exposure. If objects in the scene are moving during capture, a longer exposure time may result in an image with object motion blur. Object motion blur is different from image capture device-induced blur which is produced when the image capture device is moving relative to the scene during capture and is present in an entire image. Methods to reduce image capture device-induced blur are well known to those in the field. One method is to use a gyroscopic device to measure image capture device motion and then use a special lens with a lens element that can be moved laterally to cause the image on the electronic image sensor in the image capture device to move in a direction that compensates for the camera motion. A second method is described in U.S. patent application Ser. No. 11/130,690 by Deever et al, this method is based on a digital shifting of images in a video capture to compensate for movement of the digital camera and reduce image capture device-induced blur and to stabilize the images in the video.
One method to reduce object motion blur is to shorten the exposure time. This method under-exposes the electronic image sensor during image capture so dark images are generated. An analog or digital gain can be applied to the image signal to brighten the dark images, but those skilled in the art will recognize that this will result in noisy images.
Another method to reduce object motion blur is to shorten the exposure time and preserve more of the light that passes through the optical path and direct it to the pixels of the electronic image sensor. This method can produce images with reduced object motion blur and acceptable noise levels. However, the current industry trend in electronic imaging systems is to make imaging systems smaller and less expensive. High-grade optical elements with large apertures, which can gather more light and preserve more light passing through them, are therefore not practicable.
Another method to reduce object motion blur is to shorten the exposure time and supplement the available light with a photographic flash. A photographic flash produces a strong light flux that is sustained for a fraction of a second and the exposure time is set to encompass the flash time. The exposure time can be set to a significantly shorter interval than without a flash since the photographic flash is strong. Therefore, the blur caused by object motion during the exposure is reduced. However, objects in bright daylight can still have motion blur and flash photography is useful if the distance between the flash and the object is small.
U.S. Pat. No. 6,441,848 issued Aug. 27, 2002 to Tull describes a digital camera with an electronic image sensor that removes object motion blur by monitoring the rate at which electrons are collected by each pixel. If the rate at which light strikes a pixel varies, then the brightness of the image that the pixel is viewing must be changing. When a circuit built into the sensor array detects that the image brightness is changing, the amount of charge collected is preserved and the time at which brightness change was detected is recorded. Each pixel value where exposure was stopped is adjusted to the proper value by linearly extrapolating the pixel value so that the pixel value corresponds to the dynamic range of the entire image. A disadvantage of this approach is that the extrapolated pixel values, of an object that is already in motion when the exposure begins, are highly uncertain. The image brightness, as seen by the sensor, never has a constant value and, therefore, the uncertainty in the extrapolated pixel values results in an image with motion artifacts. Another disadvantage is that it uses specialized hardware so it cannot be used with the conventional electronic image sensors that are used in current commercial cameras.
Thus, there exists a need for producing images with reduced object motion blur, by using conventional electronic image sensors, without the use of a photographic flash, and without increasing image noise.